The present invention relates to BiCMOS and bipolar track and hold amplifiers.
The ability to track and then sample an analog waveform is often an essential function in the signal acquisition interfaces for digital video systems and high speed instrumentation. In such interfaces, analog signals must often be samples and stored accurately prior to quantization by either sequential or parallel means. In sequential A/D converters, the quantization is accomplished in two or more clock cycles using a pipelined architecture. Thus, the analog input must be sampled and then held for several clock cycles to within the accuracy specified for the overall system. In fully parallel converters, the use of a track and hold amplifier significantly reduces the aperture and sampling uncertainties of the conversion, while at the same time easing the performance demands on the subsequent quantizer.
Track and hold amplifiers are also used to provide deglitching following digital-to-analog conversion. In video D/A converters, significant glitch energy may appear at the analog output, usually as a result of mismatch in the turn-on and turn-off of switches at major bit transitions. In applications such as CRT display drivers and signal generation in automatic test systems, it is often essential that D/A converter output transitions be monotonic and as smooth as possible.
High-performance track and hold circuits have traditionally been implemented in hybrid technologies that provide components such as open transmission lines for nearly 100% sampling efficiency, step recovery diodes for fast switching transitions, and low parasitic Schottky-diode bridges for use as sampling switches.
Hybrid implementations, however, are quite costly and complex with respect to monolithic implementations. It would be very desirable, therefore, to provide an improved BiCMOS track and hold amplifier which can be incorporated into a monolithic implementation.